Land grid array structures and methods for engineering change

ABSTRACT

A Land Grid Array structure is enhanced with a flex film interposer that not only provides a Land Grid Array (LGA) electrical connection between a Multi-Chip Module (MCM) and the next level of integration such as a system board, but also provides a reliable means to implement desired Engineering Change (EC) capability as well as a means for decoupling power to ground structure to minimize switching activity effects on the System. The invention as described can be implemented for EC repair, for Capacitive Decoupling or both, using a removable and restorable engineering change plug.

RELATED APPLICATIONS

This application is a continuation-in-part and division of U.S. Ser. No.10/202,726 filed Jul. 7, 2002, now U.S. Pat. No. 6,954,984 issued Oct.18, 2005, and U.S. Ser. No. 10/775,922 filed Feb. 10, 2004, both ofwhich cases are entitled Lane Grid Array Structures and Methods forEngineering Change”.

FIELD OF THE INVENTION

This invention relates to Land Grid Array (LGA) compression connectors,and particularly to a method using LGA connectors for executingengineering changes (EC wiring) to both repair and/or implementfunctional changes to System boards with improvements in the LGAstructure.

Trademarks: IBM® is a registered trademark of International BusinessMachines Corporation, Armonk, N.Y., U.S.A. Other names may be registeredtrademarks or product names of International Business MachinesCorporation or other companies.

BACKGROUND

Before our invention a Land Grid Array structure has been made in theform of Land Grid Array (LGA) compression connectors by developed byTyco® (Tyco electronics is a division of Tyco International Ltd) andCinch® (Cinch Connectors, Inc., a part of Snecma, France), and we havefound engineering changes have proved difficult with these devices.

We found that as higher density connectors are implemented on Systems,there is a need for interconnect schemes that permit significant I/Odensities while at the same time providing an excellent signal integrityinterconnection to the next level of packaging. With the advent of theLand Grid Array (LGA) structures using compression connectors asdeveloped by Tyco, Cinch and others, the density and signal integrityproblems have been addressed. However, this structure has caused a newproblem to surface. Specifically, there is a need for a suitable methodfor executing engineering changes (EC wiring) to both repair and/orimplement functional changes to System boards. In the early stages ofhardware build, there are times that engineering changes are needed forthe System boards. These modifications are due to a number of reasons,including architecture changes, additional functional requirements, orearly user availability of imperfect hardware.

SUMMARY OF THE INVENTION

A Land Grid Array structure is enhanced with a flex film interposer thatnot only provides a Land Grid Array (LGA) electrical connection usingPlated Through Holes in the flex film between a Multi-Chip Module (MCM)and the next level of integration such as a system board, but alsoprovides a reliable means to implement desired Engineering Change (EC)capability as well as a means for decoupling power to ground structureto minimize switching activity effects on the System. In accordance withan embodiment of the invention for EC repair an insulating plug isapplied to remove an original board net and to prevents contact to anI/O pad which in turn was connected to the original board net. Theinvention as described can be implemented for EC repair and forCapacitive Decoupling.

These and other improvements are set forth in the following detaileddescription. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Land Grid Array (LGA) structure using a multi-layerflex film structure (30) composed of two conductive reference planes(31,32), two signal layers (36,33), and a plurality of our providedPlated Through Holes (herein PTH) that are located between theMulti-Chip Module MCM (40) (or similar type chip carrier) and the Systemboard (50) of the Land Grid Array (LGA) structure.

FIG. 2 details the improved structure for an LGA interposer and alsoshows a new alternative to the injection method for the electricalconnection.

FIG. 2B illustrates the use of an insulated plug to isolate theconnection from the MCM to the LGA connector.

FIG. 3 illustrates the embodiment for preferred EC (Engineering Change)repair structure for a Cinch connection scheme where repair is initiatedby the removal of an original fuzz button (35) and the installation ofan insulated polymer plug into the PTH (34A) from the System board (50).

FIG. 4 illustrates EC repair structure for an improved representativeTyco LGA connector structure.

FIG. 4B illustrates a self contained EC repair structure with connectionstrap as part of the repair structure.

FIG. 5 further illustrates the EC structure repair.

FIG. 5B further illustrates the EC structure with self containedconnection strap.

FIG. 6 shows a cross sectional view of a completed EC repair to a sparesystem board net for a Cinch connection scheme.

FIG. 7 illustrates our method for providing decoupling of the powersupplies.

Our detailed description explains the preferred embodiments of ourinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a flex film interposer that not only provides aLand Grid Array (LGA) electrical connection between a Multi-Chip Module(MCM) (or similar integrated circuit type structure) and the next levelof integration such as a system board, but also provides a reliablemeans to implement Engineering Change (EC) capability as well as a meansfor decoupling power to ground structure to minimize switching activityeffects on the System. The invention as described can be implemented forEC repair, for Capacitive Decoupling or both. Hence, the overall valueof the invention is highly enhance by this flexibility of conceptapplication.

The LGA structure with the compression connector scheme requires anelectrically insulated backing structure to counter balance the forcesused to compress the chip carrier to the System board. In a typicalapplication, this arrangement requires an insulating sheet/plate and ametallic stiffening plate. By nature of the earlier designs, theassembly prevented the access to the connector area on the board andhence prevented access to the I/O's of the chip carrier. In mostapplications, this was permissible. However, when engineering changesare needed to the DC and high frequency connections, the ability tomaintain excellent electrical measurement characteristics is compromisedand thus our new method to accommodate EC changes is required. Inaddition, as the trend continues to drive the packaging of circuits inhigher densities and smaller areas, there is a growing need to providelarge amounts of power supply capacitive decoupling as close to the chipcarrier as possible.

Our Solutions:

1) Both EC Repair & Capacitive Decoupling

As shown in FIG. 1, a multi-layer flex film structure (30) is composedof two reference planes (31,32), two signal layers (36,33), and aplurality of our provided Plated Through Holes (herein PTH) (PTHs aregenerally 34 and shown in the sections illustrated at 34A, 34B) that arelocated between the Multi-Chip Module MCM (40) (or similar type chipcarrier) and the System board (50). For the Cinch connection scheme, afuzz button (35) is inserted in the PTH (34A), thus providing theelectrical connection between the MCM (40) and the System board (50) viathe contacting pads (41) and (51). For PTHs that are connected to poweror ground locations on the System board (50) and MCM (40), thecorresponding power or ground plane within the flex film is connected tothe respected PTH. In the example shown in FIG. 1, reference plane (31)is connected to PTH (34A) and reference plane (32) is connected to PTH(34B). In addition, the use of plated through holes (34A, 34B) in theflex carrier (30), allows for more areas of electrical contacts withinand to the fuzz button (35) structure from the MCM (40) to the systemboard (50) side of the interposer. The use of PTHs in the flex filmcarrier will reduce the overall resistance of the connections as well asincrease their overall reliability. FIG. 2 details the improvedstructure of the Tyco LGA interposer on the left side of the FIG. 2 andour improvement in accordance with our preferred embodiment, thisdesign, on the right side of FIG. 2. Normally, the electrical connectionis provided by an injected connecting conductor (60) made fromconductive polymer (e.g., sliver impregnated silicone) that is injectedinto a PTH (34A). FIG. 2 also shows a new alternative to the injectionmethod for the electrical connection. In this case, the connector (60)is composed of two polymer halves (63, 64) that are riveted togetherinto the PTH (34B) by a polymer or other suitable material rivetexpansion pin (65). In an example of our preferred embodiment, thisrivet expansion pin (65) is an integral part of the base part (63) halfof the polymer conductor (60). Again, the use of plated through holes(34A, 34B), reduces the resistivity of the electrical connection as wellincreases its overall reliability. In addition, the riveting techniquewill also provide a means for simplified modifications of a signalconnection when used with the proposed EC application.

Another possibility is to electrically isolate the MCM at a location byusing the insulating plug (65) & (70) and an insulated plug (64B) onboth sides of the LGA (30) PTF hole (34B) as shown in FIG. 2B. This isuseful if a circuit needs to be temporarily removed from the system, butat a later time, the net could be restored without significant rework.This validates the rework claims as well as another aspect for the useof the insulating plug structure (65) & (70)

In both FIGS. 1 and 2, a capacitive structure is formed between theconductive reference planes (31) and (32). The reference plane (31) isconnected to PTH (34A) and reference plane (32) is connected to PTH(34B). This provides for a highly decoupled power distribution structureunder the MCM (40).

2) Capacitive Decoupling Only

A simpler method for providing decoupling of the power supplies can beaccomplished by the example illustrated in FIG. 7. In this example, theFlex Film (30) contains two or more conductive reference planesconnected using a structure described for the Cinch type connectorscheme. In this case, the reference plane (31) is connected to the PTH(34A) and likewise the reference plane (32) is connected to another PTH(34A). By doing so, a highly decoupled power supply structure is formedwithin the flex film carrier.

Another application of that application described in FIG. 1 would be tointerchange the reference and signal planes within the flex film carrierand reducing the dielectric thickness between the reference planes. Inaddition, the dielectric constant of only the insulator between theconductive reference planes could be increased as high as necessary. Bydoing this, the capacitive decoupling between the reference planes isincreased higher then that for the case illustrated in FIG. 1. Theadvantage for doing would be seen when the requirement for decoupling ofthe supplies exceeds the requirements for the signal integrity, theparameters of the EC repair and that the higher dielectric constant didnot adversely impact the signal integrity of the nets that passedthrough the flex film.

3) EC Repair Only

The formation of the EC repair structure for the Cinch connection schemeis shown in FIG. 3. Repair is initiated by the removal of the originalfuzz button (35) and the installation of an insulated polymer plug intothe PTH (34A) from the System board (50) side of the carrier. This plugis composed of two parts, the first being the plug body (70) and thesecond being the expansion pin (65). The expansion pin (65) helps tosecure the insulating plug (70) into the PTH (34A). The thickness of theplug body (70) where it contacts the pad (51) is adjusted to becompatible with the normal separation between the carrier (30) and thesystem board (50). This insulator plug will then remove all conflicts ofthe system board net with the EC repair. Next, a replacement fuzz button(35B), which is smaller than the original button (35), is inserted intothe PTH (34A). Completion of the EC is accomplished by using a smallmetal jumper (80) between the top surface metal of the PTH (34A) and thecorresponding terminal pad (100) of the X (33) and Y (36) wires that arewithin the flex film carrier (30).

The method for EC repair structure for a representative Tyco structureis shown in FIG. 4. Repair is initiated by the removal of the originalpolymer structure (60). This is accomplished in one of two methods. Ifthe polymer structure was formed by injection, the signal contact isremoved by cutting the crown (66) off and pushing the remainingstructure out of the PTH (34B). If the structure was formed by themethod of a rivet, then the rivet expansion pin (65) is removed and theconductive segment (63) is removed. Then an insulated polymer plug (70)is inserted into the PTH (34B) from the system board (50) side of thecarrier and a replacement cap (64) is inserted and held with a pin (65).Completion of the EC is accomplished by using a small metal jumper (80)between the top surface metal of the PTH (34A) and the correspondingterminal pad (100) of the X (33) and Y (36) wires within the flex filmcarrier (30).

As illustrated in FIG. 4B, a connection to the terminal pad (100) isaccomplished by a REMOVABLE and RESTORABE EC so that the originalelectrical connection could be restored and not damage the LGA (30)structure but at the same time permit good electrical connections forthe change. After the metallic cap (64A) is inserted into the PTF (34B)and aligned to the direction the EC is to take place, an insulated plugstructure (65) & (70) is inserted into the opposite side of the PTH(34B) of the LGA (30) structure. The tab (82) then contacts the feature(100) and completes the connection between the metallic plug (64A) andthe top surface pad (100), Thereby completing the EC change.

A further explanation of the method for creating an EC structure can beseen in FIG. 5 and the following text. A signal at location (34A) isremoved from the system board net by one of the proceeding methods. Tofurther the repair, a top surface structure is formed around each signallocation. Four terminal patterns (100) are placed on the sides of thesignal PTH (34A) and are connected to the imbedded X (33) and Y (36)signal repair wires through the Vias (83). In addition, a turnabout pad(110) is placed in the corner area formed by the X and Y placement ofthe terminal land pattern (100). A metal jumper (80) is formed either byplating, screening, sputtering, or by discrete wire. A deletion on theland pad (100) is made at the notched location (115). The deletion atthis point can be made by mechanical removal of the notch area or bylaser cutting. This removes the unwanted imbedded X or Y wire from therepair. To change directions within the repair, jumpers (116A, 116B) areused to connect the terminal land pattern (100A, 100B) to the turnaboutpad (110). Thus by using the X and Y wires with turnabout pads, repairscan be wired out in any direction of a high density array of connectionsto awaiting spare nets outside of the MCM placement.

With the integration of the connector strap (80) is part of the cap(64A) another method for the EC is possible. When this cap (64A) is usedin conjunction with the insulated plug (65) & (70) shown in FIG. 5A, amethod for returning the LGA structure back to an un-modified state ispossible. Since the insulator plug 65) & (70) and the electricalconnection (64A) can be removed,-the EC change can be nullified and thesystem can be returned to its original state with little effort.

A cross sectional view of a completed EC repair to a spare system boardnet is shown in FIG. 6. It can be seen that the Cinch EC repair isapplied to remove the original board net (300), although the EC repaircould be applied to a structure made with the Tyco approach previouslydescribed. The insulating plug (70) prevents contact to the I/O pad (51)which in turn was connected to the board net (300) by via (320). Thenet's path is now from the PTH (34A) through the connection strap (80)to the top surface land pattern (100). Continuing from this pad (100)through to the via (83) to the imbedded wire X wire (33) which is thenconnected to another PTH (34A). This PTH is located outside of the MCM(40) area and is under part of the MCM retaining fixture (200). Toprevent shorting of the fuzz ball (35) to the retaining structure aninsulator (210) is installed between the flex film carrier (30) and theretaining fixture (200). Completion of the repair or EC is made byconnection to the spare net (310) by the fuzz button (35) by compressingthe stiffener retaining structure (200) to finish the net.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A method of making an engineering change for a system boardstructure, comprising the steps of: providing a system board, providinga plurality of chip carrier modules for a next level of integrationprovided by a system board, at least one of which plurality of chipcarrier modules has a functional connection that could be desirablychanged, and providing a flex film carrier between the system board andthe chip carrier modules that fulfills a desired engineering change byphysically changing an original connection to another desired connectionbetween said chip carrier modules that could be desirably changed andthe system board, wherein said physically changing original connectionincludes of inserting one or more removable plugs on the system board tochange said original connection to another desired connection toaccomplish said physical changing of the original connection to theanother desired connection between the chip carrier modules and thesystem board.
 2. A method of making an engineering change for a systemboard structure, comprising the steps of: providing a plurality of chipcarrier modules for a next level of integration provided by a systemboard, at least one of which plurality of chip carrier modules has afunctional connection that could be desirably changed, and providing aflex film carrier between the system board and the chip carrier modulesthat fulfills a desired engineering change by physically changing anoriginal connection to another desired connection between said chipcarrier modules that could be desirably changed and the system board,wherein said physically changing an original connection includesinserting one or more removable plugs applied to remove an originalboard net and to prevent contact to an I/O pad which in turn wasconnected to the original board net.
 3. A method of making anengineering change for a system board structure, comprising the stepsof: providing a system board, providing a plurality of chip carriermodules for a next level of integration provided by a system board, atleast one of which plurality of chip carrier modules has a functionalconnection that could be desirably changed, and providing a flex filmcarrier between the system board and the chip carrier modules thatfulfills a desired engineering change by physically changing an originalconnection to another desired connection between said chip carriermodules that could be desirably changed and the system board, whereinsaid step of physically changing an original connection includes makingchanges with a removable connector plug composed of two polymer halvesthat are riveted together into plated through holes in said flex filmcarrier by a conductive rivet expansion pin which forms part of a basehalf of the conductor making the connection through the plated throughholes which reduce the resistivity of the electrical connection.
 4. Themethod of making an engineering change for a system board structureaccording to claim 3, wherein said step of making changes with aconnection composed of two said two polymer halves includes insertion ofan insulating part of said two polymer halves that are riveted together.5. The method of making an engineering change for a system boardstructure according to claim 3, wherein making said desired connection atop surface structure is formed around each signal location includingfour terminal patterns placed on the sides of plated through holesforming signal plated through holes which are connected to imbedded Xand Y signal repair wires through the plated through holes which formvias and a turnabout pad is placed in the corner area formed by an X andY placement of the terminal land pattern.
 6. The method of making anengineering change for a system board structure according to claim 5wherein in making a change in direction for said desired connectionmetal jumpers are used to connect the terminal land pattern to theturnabout pad.
 7. The method of making an engineering change for asystem board structure according to claim 6, wherein, while making achange, a repair is applied to remove an original board net with saidremovable connector plug preventing contact to a board I/O pad.
 8. Themethod of making an engineering change for a system board structureaccording to claim 7, wherein in making said change to prevent shortingof a fuzz ball to the retaining structure an insulator is installedbetween the flex film carrier and the retaining fixture and completionof the repair or engineering change is made by connection to a spare netby a fuzz button.
 9. A method of making an engineering change for asystem board structure according to claim 1, wherein said physicallychanging an original connection includes initiating repair engineeringchange to change said original connection to another to accomplish saidphysical changing of the original connection to another desiredconnection between the chip carrier modules and the system board by theremoval of the original polymer structure passing through plated throughholes of via(s) of a desired connection change and then a removablepolymer plug is inserted into the plated through holes being changedfrom the system board side of the carrier and a replacement cap isinserted and held with a pin.
 10. The method of making an engineeringchange for a system board structure according to claim 9, wherein saidstep of physically changing an original connection includes the step ofcompletion of a repair or engineering change by using a small metaljumper between the top surface metal of the plated through hole viabeing changed and a corresponding terminal pad of X and Y wires withinthe flex film carrier.
 11. A method of making an engineering change fora system board structure according to claim 1, wherein said step ofphysically changing an original connection includes initiating repair orengineering change to change said original connection to another toaccomplish said physical changing of the original connection to anotherdesired connection between the chip carrier modules and the system boardby removal of an original fuzz button and installation of a removablepolymer plug into plated through holes for the original connection beingchanged from the system board side of the carrier.
 12. The method ofmaking an engineering change for a system board structure according toclaim 11, wherein said insulating polymer plug is composed of two parts,the first being the plug body and the second being an expansion pinwhich helps to secure the removable plug into the plated through holeand adjusting the thickness of the plug body where it contacts aterminal pad to be compatible with the normal separation between thechip carrier modules and the system board to remove conflicts of thesystem board net with the repair or engineering change.
 13. The methodof making an engineering change for a system board structure accordingto claim 12, wherein a a replacement fuzz button which is smaller than aoriginal button at said plated through hole being changed is insertedinto the plated through hole and completion of the repair or engineeringchange uses a metal jumper between the top surface metal of the platedthrough holes and the corresponding terminal pad of X and Y wires thatare within the flex film carrier.
 14. A method of making an engineeringchange for a system board structure according to claim 1 wherein anoriginal connection normally is provided by a conductor made from twoconductive polymer halves that are riveted together into the platedthrough holes with a conductive expansion rivet pin.
 15. The method ofmaking an engineering change for a system board structure according toclaim 14, wherein the rivet pin is an integral part of the base parthalf of the conductive polymer conductor to create a capacitivestructure is formed between conductive reference planes of said flexfilm which provide for a highly decoupled power distribution structureunder the chip carrier.
 16. A method of making an engineering change fora system board structure according to claim 1 wherein said flex filmcarrier contains two or more conductive reference planes connected usinga structure with plated through holes in said Land Grid array structurewith one reference plane being connected to one plated through hole andlikewise another reference plane is connected to another plated throughhole to form a highly decoupled power supply structure is formed withinthe flex film carrier.
 17. The method according to claim 1 wherein aconnection to a terminal pad is accomplished by said removable plug andsaid removable plug is restorable so that the original electricalconnection could be restored and not damage the board structure but atthe same time permit good electrical connections for the change.
 18. Themethod according to claim 1 wherein said removable plug includes a tabwhich completes the connection to the another desired connection tocomplete the engineering change.
 19. The method according to claim 1where said removable plug is an insulator plug and the another desiredconnector can be removed and the EC change can be nullified and thesystem can be returned to its original state.
 20. The method accordingto claim 1 wherein the removable plug electrically isolates the multichip module MCM at a location by using an insulating plug and aninsulated plug on both sides of a land grid array hold if a circuitneeds to be temporarily removed from the system, but at a later time,the net can be restored without significant rework.